We grafted childhood B-precursor acute lymphoblastic leukemia (ALL) bone marrow (BM) cells into mice with severe combined immunodeficiency (SCID), in order to study the clonal evolution of immunoglobulin heavy chain (IgH) rearrangements in the absence of selective pressure by chemotherapy. BM cells from nine patients (six diagnosis samples and three relapse samples) were intravenously injected into SCID mice (three mice for each patient). All mice injected with cells from four patients developed a leukemia-like illness 12-40 weeks after injection. By PCR, new subclones that were the result of ongoing IgH rearrangement according to the mechanism operative in the injected cell populations (VH-replacement or VH to D-JH joining) were detected in the engrafted cell populations for all four patients. Subclones were mouse-specific, suggesting that subclone formation is a continuous process. Southern analysis after engraftment was unaltered as compared to the injected cells for one patient and revealed changes indicative of altered clonal composition for three patients. For two patients the observed changes possibly reflect the initial engraftment of a limited number of cells and occurred without changes in other parameters of the engrafted cell population, such as time needed for the development of leukemia, macroscopic organ involvement, immunophenotype and S-phase fraction. In one patient, we demonstrated the selective outgrowth of only a single cell type present at diagnosis, as characterized by IgH rearrangements. Our data show that evolution of clonal IgH rearrangements in B-precursor ALL may occur without the selective pressure of chemotherapy. Additionally, in some patients subclones present at diagnosis, as defined by IgH rearrangements, also possess different biological properties.